In the past, various methods have been studied for non-destructive estimation of the leaf areas of tree, which is one index of tree production capacity; one of these is a method in which the total tree leaf area is estimated by measuring light transmitted by tree leaves using an optical tree structure measurement device. Here, an optical tree structure measurement device is placed in positions below isolated trees or below a plurality of trees existing at intervals, light transmitted by tree leaves is measured, and the data obtained is used to estimate the total leaf area of tree to evaluate tree production capacity.
The following references disclose using a Plant Canopy Analyzer (PCA, a product name) as an optical tree structure measurement device to estimate the leaf area of trees.
Non-patent Reference 1
Takayuki Nakano, “Application of Plant Canopy Analyzer for Mature Tea (Camellia sinensis L.) Bush” (Japanese Journal of Crop Science, 69(3): 419-423 (2000))
Non-patent Reference 2
J. S. Broadhead et al, “Comparison of method for leaf area in tree rows”, Agricultural and Forest Meteorology, 115:151-161 (2003))
In Non-patent Reference 1, a Plant Canopy Analyzer is applied to diagnosis of the leaf area of rows of tea bushes which constitute an isolated plant canopy. During measurements, the PCA is directed toward the bush center portion, a view cap with an aperture of 90° was mounted in order to regulate the field in the azimuthal direction (horizontal direction), and a field-regulating filter is mounted in order to regulate the field in the zenith angle direction from 0° to 60°. In this method, estimating leaf area, calculations are performed assuming a uniform population and in the case of tea bushes the quantity of branches is large compared with leaf groups, so the leaf area index is over-evaluated, thus making use in leaf area estimation difficult.
In Non-patent Reference 2, a Plant Canopy Analyzer was applied to estimation of the leaf area density (ratio of the area of leaves per unit volume) for rows of trees (Croton megalocarpus, Melia volkensil) in a savanna in Kenya, and assuming the cross-section of tree rows to be elliptical and using the estimated optical path lengths, the leaf area density was calculated based on a method of calculation for isolated trees. Using this method, the correlation between calculation results and actually measured leaf area densities is low, and consequently the zenith angle distribution of inclination angles for individual leaves is newly measured, and the correlation between calculation results obtained from a model based on this and measured values is heightened. However, because a new model is added, considerable time and effort are required to measure the inclination angles of individual leaves, and so this cannot be said to be a practical method.
A PCA (Plant Canopy Analyzer: U.S. LI-COR model LAI-2000) is a representative optical tree structure measurement device currently in use. As shown in FIG. 1, this device comprises a lens system including a fisheye lens which captures light within an incident angle range of 148° at the tip end, a reflecting mirror, a filter and a detector. In the detector, photosensitive elements of silicon are disposed in a concentric circular array, so as to detect light with five different zenith angles. Data obtained in measurements by the detector is processed and control of data transfer to a computer is executed.
The PCA is generally used for measurements of plant canopies, which have a radial-direction spreading of three or more times the height z of the population and which have a uniform leaf area distribution and a uniform height according to the azimuthal directions with a cross-sectional shape such as that shown in FIG. 2. In this case, the optical path length S(θ) at the zenith angle θ, at which the transmissivity of a population is measured on the earth's surface, is estimated using the following equation.S(θ)=z/cos θ
However, in the case of an isolated plant canopy (a single isolated tree or similar) for which the assumption of the above equation is not adapted, in general there are a need for measurement of transmitted light with the sensor at the base of the trunk and oriented toward the outside of the plant canopy and also a need for measurement of optical path length at zenith angle θ at the measurement point. Because the shape of an isolated plant canopy and the leaf area are not uniform but variable depending on the azimuthal direction, it is necessary to make measurements in approximately four azimuthal directions, and consequently length measurements must be performed in a total of 20 cases (five optical path lengths×four compass directions). Further, because the sensor is directed outward of the plant canopy, the middle upper portion of the population is not subjected to measurements. Also, because what is obtained in this method is the leaf area density (the leaf area per unit volume of tree), measurements for the volume of the plant canopy must be performed as another step in order to calculate the total leaf area.
The inventor measured transmitted light for tree of 33 citrus trees using a PCA and, assuming a plant population with a substantially uniform leaf area distribution and height with azimuthal direction as described above, estimated tree leaf areas. As a result, the relation between the leaf area index (the ratio of the leaf area per unit area) as measured by a PCA and the tree crown leaf area index (the ratio of the leaf area to the area of the tree crown projected onto the ground) is shown in FIG. 3, where the correlation is low (r=0.418) with considerable dispersion regardless of the magnitude of the leaf area density. Conceivable reasons for this low correlation between the leaf area index measured by PCA and the tree crown leaf area index include the fact that the citrus trees measured are not a tree canopy having uniform height, for which a PCA is originally intended.
A conventional method of tree crown leaf area estimation using an optical tree crown structure measurement device cannot be applied to trees having only old leaves (leaves which have lasted through winter) with low leave area densities, and measurement is often difficult, and there has also been such a problem that the ratio of new leaves (newly developed leaves which have not lasted through winter) to old leaves cannot be evaluated. Further, when estimating the tree crown leaf area of trees which are isolated or which exist at intervals and do not constitute a tree group with uniform height, much time is required for estimation, and inevitably there is only low correlation between the leaf area index obtained as a result of measurement and the tree crown leaf area index. Hence it has been sought to execute estimation without requiring much time, to heighten the correlation between the leaf area index obtained as a result of measurement and the tree crown leaf area index and to reduce required expenses.